While 21 will pre-build most of the test for you, it is likely that you'll want to customize your test. What can you do?
Navigate to the right screen, tap on it or click "Add Action". Choose the element on the screen or through the object tree.
Simply hover on the element and click on it.
If the element is not selectable from the screen you can always choose it from the object tree: Simply click on "Object tree" and navigate to your desired element. Use the object tree for noisy screens that include many elements. Choose your element wisely. An element that has IDs will process faster than elements that don't.
To set data simply choose "Set Text" from your menu and choose the element you want to set the text to If you choose a TextView you have a number of options to set the text. Read "Setting your data" for the full details.
Once done click on "Add Action" and you're done :-)
Using Computer vision
In some scenarios the object tree might not expose the text or will the text in the object tree will be different than the one rendered. Use computer vision for such instances.